Regarding the shotgun doing all kinds of collateral damage... it won't .

At home defense distances, those little pellets don't spread very far at all.... rather tight, actually.

I had a friend flash me with a 100 lumen flashlight at night. I was blind for well over a minute... utterly helpless. If I were to try to shoot at the light, I would be required to look even more closely at that light (to take aim)... something that would be very difficult.... especially considering that the dude with the flashlight already has me in his sights.

One nice advantage of having handgun rounds in a carbine is that you can have a very simple arsenal without any confusion. 9mm glocks combined with 9mm Sub 2000 gives you one type of ammo... one type of magazine.... and the carbine greatly improves the characteristics of the 9mm.... gets it close to a 357 magnum.